Add a Resolver interface

[jsha]

Right now ureq will always use to_socket_addrs to lookup hostnames. For testing, it would be useful to force all lookups to resolve to localhost and a test server running there. This is particularly useful when a variety of hostnames are needed, or when testing large transfers that would be expensive to send over a real network.

We should add an internal Resolver interface that uses to_socket_addrs by default, but can be overridden during testing to provide mocked-out results.

[lolgesten]

@jsha in hreq i got this https://docs.rs/hreq/0.3.0/hreq/trait.RequestBuilderExt.html#tymethod.with_override

Would something like that work here too? it effectively makes it so the entry URI is masked by the override host/port/tls setting. That also works on redirects, so if you follow a redirect chain:

URI1 -> URI2 -> URI1

also when you get redirected back to URI1, the override kicks in.

[jsha]

Yep, the with_override method looks like a great solution! My inclination would be to enforce that it's test-only via #[cfg(test)] until someone comes along with a use case that strongly motivates making it public.

[lolgesten]

Sure! Let's gate.

[rawler]

I have a use-case for this in a production-environment, where I'd like to ensure the "spread" of requests are even between hosts. (Basically round-robin). I'll whip up a PR and let's see what you think.